In the manufacture of paper on continuous papermaking machines, a web of paper is formed from an aqueous suspension of fibers (stock) on a traveling mesh papermaking fabric and water drains by gravity and suction through the fabric. The web is then transferred to the pressing section where more water is removed by pressure and vacuum. The web next enters the dryer section where steam heated dryers and hot air completes the drying process. The paper machine is, in essence, a water removal, system. A typical forming section of a papermaking machine includes an endless traveling papermaking fabric or wire, which travels over a series of water removal elements such as table rolls, foils, vacuum foils, and suction boxes. The stock is carried on the top surface of the papermaking fabric and is de-watered as the stock travels over the successive de-watering elements to form a sheet of paper. Finally, the wet sheet is transferred to the press section of the papermaking machine where enough water is removed to form a sheet of paper.
Paper is generally made of three constituents: water, wood pulp fiber, and ash. “Ash” is defined as that portion of the paper which remains after complete combustion. In particular, ash may include various mineral components such as calcium carbonate (CaCO3), titanium dioxide (TiO2), and clay (a major component of clay is SiO2). Paper manufacturers use fillers such clay, titanium dioxide and calcium carbonate to enhance printability, color and other physical characteristics of the paper. Because of its low cost, paper manufacturers are also adding gypsum (CaSO42H2O) as filler, especially in combination with calcium carbonate. The dihydrated water is commonly referred to as “crystal” water. Gypsum loses its associated water molecules when it heated to a temperature of about 200° C.
It is conventional to measure the moisture content of sheet material upon its leaving the main dryer section or at the take up reel employing scanning sensors. Such measurements may be used to adjust the machine operation toward achieving desired parameters. One technique for measuring moisture content is to utilize the absorption spectrum of water in the infrared (IR) region. A monitoring or gauge apparatus for this purpose is commonly employed. Such an apparatus conventionally uses either a fixed gauge or a gauge mounted on a scanning head which is repetitively scanned transversely across the web at the exit from the dryer section and/or upon entry to the take up reel, as required by the individual machines. IR moisture measuring devices do not distinguish “free” water that is present in paper products from “crystal” water, in other words, IR moisture measurements yield a moisture content that is the sum of the free water and crystal water. It is desirable to obtain on-line measurements of the free water content.
The total amount of ash in paper and the composition of the ash are controlled by setting the rates of flow of gypsum and other ash components as well as the flow of wood pulp fiber and water to the papermaking system. The resulting sheet is periodically sampled and burned in the laboratory to determine the composition and amount of ash in the sheet. In the laboratory, the paper is burned under predetermined conditions and the resulting ash is accurately weighed and chemically analyzed. The papermaking parameters can then be altered based upon the resulting measurements. However, this procedure of manual control suffers from the main disadvantage that it is time consuming, even when the gypsum is the only ash component used. Thus, large quantities of paper which do not meet specifications may be manufactured while the laboratory tests are being conducted. The art is in search of improved on-line moisture sensing techniques for measuring the free water content of paper products that include gypsum.